Silicon integrated angular rate sensor

ABSTRACT

A motion sensor in the form of an angular rate sensor and a method of making a sensor are provided and includes a support substrate and a silicon sensing ring supported by the substrate and having a flexive resonance. Drive electrodes apply electrostatic force on the ring to cause the ring to resonate. Sensing electrodes sense a change in capacitance indicative of vibration modes of resonance of the ring so as to sense motion. A plurality of silicon support rings connect the substrate to the ring. The support rings are located at an angle to substantially match a modulus of elasticity of the silicon, such as about 22.5 degrees and 67.5 degrees, with respect to the crystalline orientation of the silicon.

TECHNICAL FIELD

The present invention generally relates to sensors, and moreparticularly relates to a silicon motion sensor, such as an angular ratesensor, and a method of manufacturing a sensor.

BACKGROUND OF THE INVENTION

Motion sensors, such as angular rate sensors, are commonly employed invarious applications to sense motion, such as angular rate. Sensors suchas these are commonly manufactured as microelectromechanical system(MEMS) devices using conventional micromachining techniques. Typically,a MEMS sensor may employ an electrically conductive micromachined plateof metal or silicon as a sensing element. Examples of such devices aredisclosed in U.S. Pat. Nos. 5,450,751; 5,547,093 and 5,872,313.

Sensors of the type described above are capable of extremely precisemeasurements. However, conventional sensors may suffer various drawbackswhich may include mismatches in the resonant frequency between primaryand secondary flexure nodes at the sensor element, and may suffer fromrelatively high mass and performance limitations. Another drawback ofsome conventional sensors is undesirable sensitivity to vibration.Therefore, it would be desirable to provide for a motion sensor andmethod of manufacturing a sensor that overcomes drawbacks of the priorart.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a motion sensoris provided. The motion sensor includes a support substrate and asilicon sensing ring formed within and supported by the substrate andhaving a flexural resonance. The motion sensor also includes at leastone drive electrode including drive capacitive plates for applyingelectrostatic force on the ring to cause the ring to resonate, and atleast one sense electrode including sense capacitor plates for sensing achange in capacitance indicative of the vibration nodes of resonance ofthe ring so as to sense motion. The motion sensor further includes aplurality of silicon support springs connecting the substrate to thering, wherein the support springs are located at an angle tosubstantially match a modulus of elasticity of the silicon supportsprings. According to a further aspect of the present invention, thesupport springs include a first spring located at an angle in the rangeof 20° to 25° with respect to the crystalline orientation of thesilicon, and a second spring oriented at an angle in the range of 65° to70° with respect to the crystalline orientation of the silicon.

According to another aspect of the present invention, a method of makinga silicon integrated sensor on an SOI substrate is provided. The methodincludes the step of providing a substrate having an insulation layer ona top surface, and providing a silicon epitaxial layer on top of theinsulation layer. The method also includes the steps of forming a firsttrench extending through the epitaxial layer and reaching the insulationlayer so as to isolate a first portion of the epitaxial layer from asecond portion of the epitaxial layer, and disposing a fill materialwithin the first trench. The method also includes the steps of formingone or more electrical components on the first portion of the epitaxiallayer, and forming one or more contacts on the second portion of theepitaxial layer. The method further includes the step of forming one ormore second trenches in the second portion of the epitaxial layer so asto provide one or more moving elements within the second portion of theepitaxial layer, wherein the one or more movable elements serve assensing elements. According to a further embodiment, the method includesthe step of forming one or more anti-stiction bumps to prevent a portionof a moving element from sticking to an adjacent feature of the sensor.

These and other features, advantages and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims and appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a top view of a micromachined angular rate motion sensoraccording to one embodiment of the present invention;

FIG. 2 is an enlarged view of section II-II of FIG. 1 illustrating thesprings connecting the ring to the substrate, according to oneembodiment;

FIG. 2A is a schematic diagram illustrating a ring having a varyingwidth according to another embodiment;

FIG. 3 is an enlarged perspective view of section III-III of FIG. 1further illustrating the electrode structure;

FIGS. 4A and 4B are schematic diagrams illustrating a two-node flexuralmotion of the ring angular rate sensor in respective primary modevibration and secondary mode vibration;

FIG. 5 is an enlarged perspective view of section V taken from FIG. 3further illustrating isolation protrusions that form anti-stictionbumps, according to one embodiment;

FIG. 6 is an enlarged perspective sectional view of the motion sensorfurther illustrating a perforated structure according to anotherembodiment;

FIG. 7 is an enlarged perspective sectional view of the motion sensorfurther illustrating a perforated compensation mass and cornercompensations, according to a further embodiment;

FIG. 8 is a cross-sectional view of a sensor device being manufacturedfollowing the initial steps, according to one embodiment of the presentinvention;

FIG. 9 is a cross-sectional view of the sensor device illustratinganother step of the method;

FIG. 10 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 11 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 12 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 13 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 14 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 15 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 16 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 17 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 18 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 19 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 20 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 21 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 22 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 23 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 24 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 25 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 26 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 27 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 28 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 29 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 30 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 31 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 32 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 33 is a cross-sectional view of the device illustrating anotherstep of the method;

FIG. 34 is a cross-sectional view of the sensor device according to afurther step of the method;

FIG. 35 is a perspective cross-sectional view of a sensor device showingan initial step of forming anti-stiction bumps according to oneembodiment of an isolation formation process;

FIG. 36 is a perspective cross-sectional view of the sensor device ofFIG. 35 illustrating another step in the method of forming anti-stictionbumps;

FIG. 37 is a perspective cross-sectional view of the sensor device ofFIG. 36 further illustrating another step in the method of forminganti-stiction bumps;

FIG. 38 is a perspective cross-sectional view of the sensor device ofFIG. 37 further illustrating another step in the method of forminganti-stiction bumps;

FIG. 39 is a perspective cross-sectional view of the sensor device ofFIG. 38 further illustrating another step in the method of forminganti-stiction bumps;

FIG. 40 is a perspective cross-sectional view of the sensor device ofFIG. 39 further illustrating another step in the method of forminganti-stiction bumps;

FIG. 41 is a perspective cross-sectional view of the sensor device ofFIG. 40 further illustrating another step in the method of forminganti-stiction bumps; and

FIG. 42 is a perspective cross-sectional view of the sensor device ofFIG. 41 further illustrating a further step in the method of forminganti-stiction bumps.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1-3, a motion sensor 10 is generally illustratedaccording to a first embodiment of the present invention. The motionsensor 10 shown and described herein in the disclosed embodiment is anangular rate motion sensor for sensing angular rate. However, it shouldbe appreciated by those skilled in the art that the sensor 10 could alsobe configured to operate to sense other motions, such as angularacceleration, and may further be configured to sense angular positionaccording to other embodiments.

As seen in FIG. 1, the sensor 10 includes a sensing element formed on asupporting wafer or substrate 12 generally shown in FIG. 1 and morespecifically shown in FIGS. 8-34. The substrate 12 is made of silicon,according to one embodiment, and has a bottom layer 100 and has aninsulation layer 102 and epitaxial layer 104 formed on top as shown inFIG. 34. The sensing element includes a ring 14 that is generallysupported by a plurality of springs 16 each having first and secondspring members (portions) that extend from a center hub or post 18 tosupport the ring 14 relative to the supporting substrate 12. In thedisclosed embodiment, the ring 14 is circular, however, other shapedrings may be employed according to other embodiments. The ring 14,springs 16 and central post 18 may also be made of silicon to form anall-silicon monolithic structure. One example of an all-siliconmonolithic structure is disclosed in U.S. Pat. No. 5,547,093, which ishereby incorporated herein by reference.

The ring 14 is shown surrounded by a number of electrode structures 20formed on the substrate 12 and spaced at 45° intervals. The electrodestructures 20 form an equiangularly spaced electrode pattern in closeproximity to the perimeter of the ring 14. The ring 14 and theelectrodes 20 are formed of an electrically conductive material,particularly silicon, enabling the ring 14 to form a capacitor withcomplimentary features, referred to as capacitive plates, when a voltagepotential is present. Some of the electrode structures 20 are configuredas drive electrodes 20A that drive the ring 14 into resonance whenenergized, while other electrode structures 20 are configured as sensingelectrodes 20B to capacitively sense the proximity of the vibrationpattern of the ring 14, which will vary due to Coriolis forces thatoccur when the sensor 10 is subjected to rotary motion about an axisperpendicular to the substrate 12.

Each of the electrode structures 20 are also configured to includebalance electrodes 20C which, when energized, match the resonant peaksof the natural flexural modes of the ring 14 by inducing electricalspring softening in the ring 14 and in the springs 16, making the deviceresponsive to Coriolis forces that occur when the sensor 10 is subjectedto rotary motion about an axis perpendicular to the substrate 12. Thebalance electrodes 20C are formed by at least one pair of movingcapacitive plates interdigitated with fixed plates and shown beingradially inward from the drive and sensing electrodes 20A and 20B, andare electrically interconnected to electrical conductors 22. Additionalbalance electrodes 24 are shown disposed equiangularly around theperimeter of the ring 14, which serve to fine balance the sensingstructure. The balance electrodes 24 are offset by 22.5° relative toelectrode structures 20 and capacitively coupled to the ring 14 andelectrically interconnected with some of the concentric conductors 22.The ring 14, springs 16 and central post 18 are electrically insulatedfrom the wafer 12, and electrically interconnected to electricalconductors 22 outside the ring 14 so as to allow an appropriateelectrical potential to be established between the moving structure andthe electrodes 20A, 20B, 20C and 24.

The motion sensor 10 is configured to detect angular velocity about thevertical axis of the ring 14, and therefore rotary movement about anaxis of a body, such as an automobile, to which the sensor 10 ismounted. In one embodiment, conditioning circuitry (not shown) is formedon and electrically interconnects with the sensing electrodes 20B, anddifferential driving and sensing techniques may be employed. One exampleof a motion sensor having suitable conditioning circuitry and employingdifferential driving and sensing techniques is disclosed in U.S. Pat.No. 5,872,313, the entire disclosure of which is hereby incorporatedherein by reference. The operational requirements of conditioningcircuitry and the driving and sensing techniques should be appreciatedby those skilled in the art, and therefore are not discussed herein indetail.

Performance of the sensor 10 may be optimized by equiangularlypositioning the electrode structures 20 around the perimeter of the ring14. It should be appreciated that at least one drive electrode isrequired, and that more preferably at least two drive electrodes 20A areemployed, with the two drive electrodes 20A offset from each other byabout 45°, according to one embodiment. However, other electrodeconfigurations are foreseeable. It should also be appreciated that atleast one sensing electrode 20B is employed, and that more preferably aplurality of sensing electrodes 20B are employed. Further, theperformance of the sensor 10 may be enhanced by increasing the number ofsensing electrodes 20B present. According to one embodiment, it shouldbe further appreciated that at least one balance electrode 20C isemployed, and that more preferable a plurality of balancing electrodes20C are employed.

The electrode structures 20 are shown in FIG. 3 generally employing abase plate or trunk 30 that extends radially from the ring 14, andmultiple pairs of opposing teeth or plates 32 extending perpendicularlyfrom the trunk 30. Because the trunk 30 and the plates 32 are physicallyconnected to the ring 14 and formed integrally with the ring 14, thetrunk 30 and plates 32 are subjected to the same vibratory movement asthe ring 14. A stop may be employed for limiting the lateral movementfrom the trunk 30, and therefore prevents the plates 32 from contactingthe surrounding electrode structure due to excessive angular and/orlinear acceleration of the ring 14. The trunk 30 and plates 32 may besubjected to thermal expansion and contraction with the ring 14 whensubjected to variations in temperatures, which may be compensated foraccording to known techniques, such as is disclosed in U.S. Pat. No.5,872,313. It should be appreciated that temperature compensation isgenerally not needed with the disclosed embodiment of the sensor 10 dueto the all silicon construction.

Each electrode structure 20 further includes an arrangement of fixedelectrodes 34 that are interdigitized with the movable plates 32. Thefixed capacitive plates 34 are fixed to the support substrate 12. Fourof the electrode structures 20 employ the outermost pairs of movablecapacitive plates to form drive electrodes and the pair of radiallyinward movable capacitive plates 32 form balance electrodes, whereas theother four electrode structures 20 employ the outermost pair of movablecapacitive plates 32 to form sensing electrodes and employ the innermostpair of movable capacitive plates 32 to form balance electrodes 20C. Themovable capacitive plates 32 are movable relative to the fixedcapacitive plates 34 such that the distance therebetween changes thecapacitive coupling. Changes in capacitive coupling between the movablesensing plates and fixed plates are detected and indicative of thesensed angular rate. With the application of Coriolis force, radialflexural motion at nodes indicates angular rate sensed by the sensor 10,while flexural motion at anti-nodes indicates motion of the ring.

The design of the ring angular rate sensor 10 in silicon as a mechanicalstructure poses challenges when the crystal plane orientation is <100>or <110>. Integrated microsensors incorporate microsensing structure(s)along with its associated drive, sense, control and calibrationelectronic circuits on the same piece of silicon substrate to realize ahigh performance system on chip at competitive cost. Due to thesignificant differences in the electrical performance of CMOStransistors realized in a <111> plane or other planes compared to theperformance of CMOS transistors realized in <100> or <110> planes,almost all CMOS integrated circuits (ICs) utilize silicon substrateorientations of <100> or <110>.

It is known that certain mechanical properties of single crystal siliconare also orientation dependent and also vary with dopant type and dopantconcentration. It turns out that the modulus of elasticity of singlecrystal silicon is also orientation dependent. The longitudinal modulusof elasticity C′₁₁(θ), in a direction forming an angle θ with <100>silicon, can be calculated by using the following equation:

C′ ₁₁(θ)=C ₁₁(cos⁴θ+sin⁴θ)+2(C ₁₂ +C ₄₄)cos²θ sin²θ

where C₁₁ is the longitudinal modulus of elasticity along the <100>direction, and C₁₂ and C₄₄ are the corresponding transverse moduli ofelasticity. For intrinsic silicon, C₁₁=1.66, C₁₂=0.639, and C₄₄=0.796.It should be noted that C′₁₁(θ) has a larger value at 45°, 135°, 225°and 315° angles than at right angle locations of 0°, 90°, 180° and 270°.

According to one embodiment shown in FIGS. 1 and 3, the ring 14 isconfigured with a substantially uniform width. According to anotherembodiment, the ring 14 may be configured with a varying width, such asshown in FIG. 2A. In this embodiment, the width of the ring 14 variesaccording to a sinusoidal waveform to provide maximum ring widths 14A at0°, 90°, 180°, and 270°, and to provide minimum ring widths 14B at 45°,135°, 225°, and 315°. By varying the width based on a sinusoidalwaveform, the mass compensator ring 14 accounts for varying modulus ofelasticity around the ring 14.

The ring angular rate sensor 10 operates on the principle of Coriolisforce. The sensor 10 measures angular velocity by monitoring theposition of nodes in its two-node flexural vibration pattern. There aretwo degenerate frequencies of equal values associated with the two-nodeflexural mode. When the ring is electrostatically driven into anelliptical-shape vibration, at least one location (primary) on the ring,the motion at this primary location is radial by nature. However, theradial motion at the 45° (secondary) locations from the primary locationis ideally zero and only tangential motions exist at these locations.Upon the application of angular rate input, the vibration pattern whichhas four nodes and four anti-nodes at eight locations on the ring 14tends to precess, resulting in energy coupling from the primarylocations to the secondary locations, resulting in radial motion at thesecondary locations. The coupling efficiency strongly depends onmatching the frequencies of the two-node flexural degeneratefrequencies. Assuming matched primary and secondary mode frequencies,the amplitude of the radial motion at secondary locations due to nodeprecession because of the applied Coriolis force is proportional to theamplitude of the angular rate input. Any difference in the frequency ofthe degenerate modes will reduce the energy coupling from the primarymode to the secondary mode, and thus reduce the sensitivity of thedevice to angular rate input.

The angular rate sensor 10 operates based on mode matching of its twoflexural modes and symmetric damping. The mode numbers of the angularrate sensor 10 are a function of ring material properties, ringdimension and the design and dimension of the supporting springs 16. Thetwo-node flexural modes have been discovered through ANSYS simulationsto be the fourth and fifth modes of the rate sensor 10, according to oneembodiment. The fourth and fifth two-node flexural mode matching isemployed to sense angular velocity, based on vibration coupling throughthe Coriolis force. In the absence of angular rate inputs, the ringvibrates and causes forced oscillation on the 0° and 90° locations. Theoscillation may be achieved using a known phase locked loop (PLL)circuit using the ring 14 as a reference frequency input and a voltagecontrolled oscillator (VCO) whose frequency is locked to the ringfrequency. In this situation, generally all the amplitude of ringvibration is in the first normal mode (0° and 90° locations), which isreferred to as the primary mode, and there is no radial motion in thesecond mode (±45° locations) which is referred to as the secondary mode.Upon experiencing rotation, the Coriolis force will cause energy to betransferred from the primary mode to the secondary mode, building upvibration amplitude in the secondary mode. The ratio of the amplitude ofthe secondary mode vibration to the amplitude of the primary modevibration may be representative of the following equation:

Secondary/primary=2ΔαΔQΔΩ/ω,

where α is the ring angular gain, Q is the quality factor, ω is thetwo-node flexural natural frequency, and the Ω is the angular velocityexperienced by the sensor 10.

An exaggerated version of the two-node flexural motion of the ringangular rate sensor 10 is illustrated in FIG. 4A in the primary modevibration and is shown in FIG. 4B in the secondary mode vibration. Ithas been discovered that the use of curved or semi-circular springs inprior art devices may result in a gross frequency mismatch between theprimary and secondary two-node flexural normal modes, rendering theconventional device generally insensitive to angular velocity input.This is at least partly due to the fact that the modulus of elasticityof single crystal silicon is orientation dependent. The motion sensor 10according to the present invention advantageously employs a springdesign that provides insensitivity to the silicon crystal orientationmodulus of elasticity variation. Specifically, in one embodiment themotion sensor 10 employs sixteen springs 16 (eight pairs) that areattached to the ring 14 at 45° increment angular locations at one endand to the fixed anchor center post 18 at the other end.

The springs 16 are illustrated in the layout of the sensor 10 in FIGS. 1and 2. As discussed with regard to the above equation, the modulus ofelasticity of silicon is equal along 0°, 90°, 180° and 270° directions.The modulus of elasticity of <100> and <110> silicon is also equal along±22.5° and ±67.5° with respect to 0° and 90° directions in <100> and<110> silicon. With particular reference to FIG. 2, each of the eightpairs of springs 16 is constructed with two straight beams, namely B1and B2. It should also be noted that beams B1 and B2 are oriented at±22.5° with respect to X- and Y-axes, respectively. This strategic beampositioning assures that all segments contributing to the constructionof each of the eight pairs of springs 16 have constant modulus ofelasticity, which simply ensures a matched spring constant for allsprings 16.

The second cause of mismatch between the two-node flexural modefrequencies is the varying modulus of elasticity around the ring 14. Thevariation of the modulus of elasticity around the ring 14 follows theaforementioned equation, which implies that the modulus of elasticity ismatched at locations of 0°, 90°, 180° and 270°. The modulus ofelasticity is also matched at locations of 45°, 135°, 225° and 315°.However, it should be noted that the modulus of elasticity exhibits aminimum value at 0°, 90°, 180° and 270° locations and maximum value at45°, 135°, 225° and 315° locations. The primary and secondaryfrequencies matching of the two-node flexural mode is determined by thematching of the spring constant amongst the springs 16 and the matchingof the spring constant at 0° and 45° locations on the ring 14. In orderto compensate for the spring constant variation around the ring 14, thewidth of the ring 14 may be varied according to the above equation asshown in FIG. 2A or by employing lumped compensating masses 50 at theinward end of trunk 30 at each of the 45°, 135°, 225° and 315°locations, as shown in FIGS. 1 and 3. Due to the designed-in structuresymmetries and matching of the modulus of elasticity within the ring 14and the springs 16, symmetric damping is assured. Symmetric dampingassures minimum sensor error due to drive amplitudes or temperature.This is advantageous for proper device performance in practicalsettings.

While the sensor 10 is described particularly as an angular rate motionsensor, those skilled in the art will appreciate that the sensor 10could also operate as an acceleration sensor, or as a position sensor.Further, four, six and eight node operation of the sensor 10 ispossible. The separation of the resonant peaks between the four, six andeight node resonant frequencies in the disclosed embodiment is very widefor the sensor 10; by nearly a factor of two. This allows for easydiscrimination between the resonant frequencies.

The sensor components, both movable and fixed, including the ring 14,springs 16, highly compliant external tethers 22, compensating masses50, interdigitated members including capacitor plates 32 and 34, supportstructures and anchors may be formed from a silicon-on-insulator (SOI)substrate along with on-board electronic compensating circuitry.According to one embodiment, the sensor components are all made ofsingle crystal silicon, and therefore generally have no thermal mismatchthat would need compensation. Further, the sensor components arefabricated in the plane of the substrate, which makes them less prone todamage than components of conventional devices. The sensor componentsare also formed and released during the same photolithographic and etchsteps, thereby decreasing the process complexity, and lowering the costsof manufacture.

The electrical contact to the ring 14 is made with highly compliantexternal tethers 22 that form electrical connections. The tethers 22eliminate the need for contact to the center hub 18 of the sensor 10,and the known problems associated with a center hub contact, such aswire bonding damage, stray capacitances, and variable capacitances fromwire bonds moving relative to each other. All of the sensor componentsare anchored to, but cantilevered from, an insulator film (oxide 102) ontop of an underlying substrate 100 as shown in FIG. 34, which isachieved through center hub 18, and/or through the highly compliantexternal tethers 22. All sensor components are otherwise free to move,according to one embodiment. This arrangement with trench isolationprovides for electrically and mechanically isolated components.

In one embodiment, a trench isolation scheme is utilized to form one ormore trenches 112 to isolate portions of the epi layer 104 shown in FIG.34, as well as the sensor components. Isolation trenches 112 may beprovided as shown in FIG. 5 and may be lined with an insulator film,such as an oxide, filled with an appropriate material such aspolysilicon 118, and then capped so that conductors can carry electricalsignals across the isolation areas. Later, other trenches 170, such asthose surrounding a delineated sensor component like the ring 14 or acompliant tether 22 are formed. The ends 40 of the filled isolationtrenches 112 protrude into the trenches 170 isolating the moving element22A as shown in FIG. 5. The ends 40 of the filled isolation trenches 112then act as anti-stiction bumps, thus preventing the lateral stiction ofthe movable sensor components, such as the ring 14 and the highlycompliant external tethers 22, and other sensor components, to the wallsof the epi layer, and the fixed sensor components surrounding them.

In addition, the components of the sensor 10, both movable and fixed,may be perforated to provide openings that allow for relatively lowmass, yet still retaining appropriate rigidity, and to aid in themanufacture of the sensor 10. In some designs, a lower mass for certainsensor components is desirable. Further, to aid in the manufacture ofseveral of the sensor components, perforations may be desirable. Sensorcomponents such as the ring 14, trunk 30 and compensating masses 50 mayall be perforated as shown in FIGS. 6 and 7. In addition, other sensorcomponents, such as tethers 22 and 22A and capacitive plates 32 mayinclude perforations 80.

In one embodiment of the motion sensor 10, corners of several sensorcomponents may be compensated to include portions of the epi layer 104,as shown by reference identifier 52 in FIG. 7. The corner compensation52 allows for process variation during the delineation and releaseprocesses.

In one embodiment, the movable capacitive drive and sense electrodes areconnected to trunk 30 whose attachment to the ring 14 is cantileveredfrom the ring 14. The cantilevered connection has two attachment pointsto the ring 14 for stability, and to allow a perforation 80 between thering 14 and the trunk 30. The perforation 80 may further aid themanufacture of the sensor. Differential drive and differential sensefurther improve the sensor vibration rejection and the temperatureresponse.

It should be appreciated that the angular rate sensor 10, according tothe present invention may advantageously be employed on a vehicle, suchas a wheeled automobile. For example, the angular rate sensor 10 may beemployed in an electronics stability control system to prevent accidentscaused by unwanted angular rotations of a moving vehicle. In doing so,the angular rate sensor 10 is used to detect the unwanted angularrotations.

Method of Manufacturing Sensor

A method of making an all-silicon integrated sensor 10 onsilicon-on-insulator (SOI) substrate, will now be described. The methodis discussed in connection with making an all-silicon angular ratesensor, such as sensor 10 described herein. However, it should beappreciated that the method may be employed to manufacture other siliconsensor devices. These devices may be made simultaneously on the samesubstrate and in any combination.

According to one embodiment, the method employs a CMOS process, modifiedto include an isolation and anti-stiction bump module, a defect gettermodule, and a sensor delineation etch and release module. The CMOScircuitry provides on-board signal processing for the fabricated sensor10. The isolation process module provides isolation between regions ofthe device silicon layer, between the sensor element and thecompensating electronics, and between components of the sensor element.Additionally, the isolation process module provides anti-stiction bumpssuch that the ring and other moveable portions of the sensor element,such as the external tethers, do not stick to portions of thesurrounding sensor. The method of making the sensor is flexible, suchthat sensors made with the process may sense different physicalquantities, such as angular rate, angular acceleration, and linearacceleration in one or more than one direction. These devices may bemade simultaneously, alone or in any advantageous combination.

The method will now be described in connection with the series ofprocessing steps, which in one embodiment are shown in the drawing FIGS.8-34. It should be appreciated that some common processing steps are notshown or described in detail herein. The process method according to oneembodiment of the present invention begins with a starting materialhaving a silicon substrate 100 on the bottom with an oxide insulationlayer 102 provided on top, and an epitaxial (epi) device layer 104 ofsilicon formed upon the layer of oxide 102. According to one example,the device layer 104 has a thickness of approximately 40 micrometers,and the layer of oxide 102 has a thickness in the range of 100 angstromsto 2 micrometers. The starting material including the underlyingsubstrate 100, the oxide insulation layer 102, and the epitaxial (epi)device layer 104 are illustrated in FIG. 8 according to one embodiment.

In the step shown in FIG. 9, an oxide layer 110 is provided on top ofthe silicon device layer 104, and mask and etching steps open portionsof the oxide layer 110. A second thinner layer of oxide 111 is thenformed in the window in the first oxide 110 on the silicon device layer104, creating recess 106. An n-type dopant 108, such as phosphorus, isthen implanted through the second oxide film 111, but not through thefirst oxide film 110, into the silicon (N-well implant) device layer104, and the silicon is then annealed and the dopant activated, such asby heating. The surface of the silicon device layer 104 is then strippedof oxide layers 110 and 111.

In the steps of FIG. 10, another oxide film 113 is then formed on thetop surface of the silicon device layer 104. The oxide film 113 ismasked and etched, and then the same mask is used to etch one or morehigh aspect ratio trenches 112 through the silicon device layer 104 tothe buried oxide layer 102 of the silicon substrate 100 as shown in FIG.10. The trenches 112 in the silicon device layer are lined with adielectric layer 114, such as an oxide, as shown in FIG. 11, and thetrenches 112 are thereafter filled completely with a conformal material118, such as polysilicon or nitride, as shown in FIG. 12. The conformalmaterial 118 is removed from the top surface of the structure, leavingthe high aspect ratio trenches 112 filled with the conformal material118 as shown in FIG. 13. This substantially planarizes the top surfaceof the silicon device layer 104, and completes the formation of theanti-stiction bumps 40 shown in FIG. 5. The oxide 113 on the top surfaceof the silicon device layer 104 is then removed, leaving the liner oxide114 and the conformal material 118 in the planarized trenches 112. Thetrenches 112 provide electrical isolation between different portions ofthe epitaxial device layer 104.

Yet another oxide film 116 is formed upon the top surface of the device,including upon the planarized top surfaces of the one or more trenches112 as shown in FIG. 14. A mask is patterned on the surface, and ap-type dopant 120, such as boron, is implanted into the silicon 104through the oxide 116 in the open windows areas of the mask. After themask is stripped, an inorganic film 122, such as nitride, is then formedon the surface as shown in FIG. 15.

Referring next to FIG. 16, a mask is patterned and the inorganic film122 is etched in the windows 124 to expose the oxide 116 in selectedareas. The mask is then stripped.

In another step, a mask may be patterned and a p-type dopant, such asboron (not shown), may be implanted into the substrate through the oxidein the open windows in the mask. This p-type dopant may serve as a fieldimplant to adjust resistivity of the surface where circuitry is to beformed. The mask is then stripped.

A mask may then be patterned and a p-type dopant, such as BF2 or argon(not shown), may be implanted into the substrate through the openwindows in the mask. This additional p-type dopant may serve as a getterfor collecting impurities, particularly away from electronics. The maskis then stripped.

Referring to FIG. 17, an additional thickness of oxide 128 is thenselectively formed in those areas of the substrate not covered by theinorganic film 122. An etch is done to remove any oxide 128 that wasformed upon the inorganic film layer 122, but leaves the most recentlyformed additional thickness of oxide largely intact. The inorganic film122 is then removed from the surface in the step shown in FIG. 18. Thisleaves the surface covered with two thicknesses of oxide 128 and 116,each thickness in different areas of the substrate surface. An oxideetch is then performed to remove the thinner of the two oxide films 116,exposing the silicon in those areas, and leaving most of the thickeroxide intact. The steps shown in FIGS. 17, 18 and 19 may be implementedusing processes collectively known as a LOCOS process.

Referring to FIG. 19, an oxide film 132 is then formed upon the exposedsilicon regions of the substrate. Little additional oxide forms on theareas of the substrate covered by the thicker oxide film 128. A p-typedopant, such as boron (not shown), is then implanted through the thinneroxide film 132 into the silicon 104. This p-type dopant adjusts thesurface dopant concentrations in select regions. The thicker oxide layer128 blocks the dopant from entering the silicon in those regions. Thethin oxide film 132 is then etched away to expose the silicon in thoseregions.

In FIG. 20, an oxide film 133 is then formed upon the exposed siliconregions of the substrate. Then a layer of polysilicon 134 is depositedupon the surface. An n-type dopant, such as phosphorus (not shown), isthen introduced into the polysilicon and is thermally activated. Thisn-type dopant dopes the polysilicon 134 to create enhanced conductivityof the polysilicon.

A mask is patterned and portions of the polysilicon film 134 are etchedto expose the oxide in selected areas as shown in FIG. 21. The mask isthen stripped. Exposed portions of oxide film 133 are then removed. Anoxide film 138 is then formed upon the surface, including on thepolysilicon. Little additional oxide forms on the areas of the substratecovered by the thicker oxide film 128. An inorganic film 140, such asnitride, is then formed on the surface as shown in FIG. 22. Then a layerof polysilicon 143 is deposited upon the surface as shown in FIG. 23. Ann-type dopant, such as phosphorus (not shown), is then introduced intothe polysilicon. The n-type dopant creates an n-type polysilicon withenhanced conductivity. A dielectric blocking film 144, such as undopedsilicon glass (USG) or boron phosphorus silicon glass (BPSG), is thendeposited on the polysilicon, and is densified/annealed.

A mask is patterned and a portion of the blocking film 144 is etchedthrough the open windows in the mask to expose portions of thepolysilicon layer. An n-type dopant, such as phosphorus (not shown), isthen introduced into the exposed areas of polysilicon, and is activated.This creates a polysilicon resistor circuit component. The blocking film144 is then removed as shown in FIG. 24.

Referring to FIG. 25, a mask is patterned and portions of thepolysilicon film 143 and nitride film 140 are etched to expose the oxide128 and 138 in selected areas. The mask is then stripped. An oxide film145 is then formed upon the surface of the polysilicon 143. Littleadditional oxide forms on the areas of the substrate covered by thethicker oxide film.

A mask is patterned and a p-type dopant, such as boron (not shown), isintroduced into selected regions of the substrate. The mask is thenstripped. The boron p-type dopant creates a circuit element, such as aresistor according to one example.

A mask is then patterned and a first n-type dopant, such as arsenic (notshown), is introduced into selected regions of the substrate. A secondn-type dopant, such as phosphorus (not shown), is introduced into thesame selected regions of the substrate. The mask is then stripped, andthe dopants are thermally activated. This step forms one of the sourceor drain areas of a transistor device (e.g., CMOS).

A mask is patterned and a p-type dopant, such as BF2 (not shown), isintroduced into selected regions of the substrate. The mask is thenstripped. The p-type dopant forms the other of the source or drain ofthe transistor device. An inorganic film 149, such as a deposited oxide,is then formed upon the surface as shown in FIG. 25. The dopant is thenthermally activated. An inorganic film 150, such as a spin-on glassand/or an oxide film, is formed upon the surface of oxide 149 as shownin FIG. 26. A thermal treatment is then performed.

Referring to FIG. 27, a mask is patterned, and the inorganic film 149and oxide film 138 and inorganic film 150 are etched in selected regions152 to expose the silicon 104. This forms contact regions 152. The samepatterned etching can be used to form contact regions on the polysiliconregions 143 and 134 to create electrical circuit pathways.

Referring to FIG. 28, a first metal film 154 is then formed on thesubstrate and within region 152 to make conductive contact with silicon104. A mask is then patterned and selected areas of the first metal film154 are etched to achieve the desired conductivity path as shown in FIG.28.

A first inorganic film, such as a deposited oxide, is then formed on thesurface. A second inorganic film, such as a spin-on glass, is thenformed on the surface. The surface is then etched to form a largelyplanar surface, and then a third inorganic film, such as a depositedoxide, is formed on the surface. The three aforementioned inorganicfilms are identified together as composite oxide layer 156 in FIG. 29and serve to provide a planar dielectric surface between conductiveelements. According to other embodiments, a chemical mechanicalplanarization (CMP) process may be employed to planarize the wafersurface after all metal and inter-level metal dielectric film sequences.

A mask is then patterned and portions of the composite oxide 156covering the first metal 154 are etched to form one or more via openings158 as shown in FIG. 30.

Referring to FIG. 31, a second metal film 160 is then formed on thesurface and in contact with the first metal 154 to fill in the viaopenings. A mask is patterned and selected areas of the second metal 160are etched. The mask is stripped, and an inorganic film 162, such as anoxynitride, is formed on the surface.

A mask is then applied to the device side (front) surface of thesubstrate, and the various films, including inorganics like oxide,nitride, oxynitride, and polysilicon, are removed from the back surfaceof the substrate, resulting in a bare silicon back surface. The mask isthen stripped.

Referring to FIG. 32, a mask is then patterned and portions 166 of theinorganic film 162 covering the second metal 160 are etched to formexposed contact pads. The mask is then stripped. A thermal treatment isthen applied to anneal the metal.

Referring to FIG. 33, a mask is patterned and portions of the inorganiclayers 138, 149, 150 and 156 covering the silicon are then etched toexpose the silicon in the regions where the sensing elements are formed.The etch may comprise isotropic or anisotropic etch methods. The mask isthen stripped.

A mask is then patterned and portions of the silicon are anisotropicallyetched, such as by a DRIE etch tool, to delineate and release the sensorelements as shown in FIG. 34. This includes forming trenches 170 whichprovide physical and electrical isolation and/or releases sensorelement(s) 172. Both vertical and lateral etching may be employed. Themask is then stripped, which completes the device fabrication.

A previously made cap wafer (not shown) may be aligned to the devicewafer, and the two wafers bonded together to provide a cap over thesensor element. The cap provides for a hermetic seal of the sensorelement, thus protecting it from physical damage due to handling,particulate contamination, and moisture, etc. The bond may entail usinga glass frit. The frit may be used over the on-board circuitry of theintegrated sensor without affecting the functionality of the circuit.

Finally, in steps after the integrated sensor is completed and capped,the bond pads are exposed, the device is tested, and the die aresingulated.

Micromachined sensor elements formed by this process may be fabricatedin the plane of the substrate. This makes them significantly less proneto damage than conventional approaches which have sensor componentsformed above the plane of the substrate.

The process is versatile in that sensors may be made with the processthat sense different physical quantities, such as angular rate, angularacceleration, and linear acceleration in one or more than one direction.These devices may be formed simultaneously on the same substrate in anyadvantageous combination. The process is simple, and does not requireburied cavities, thus preventing the process complexities, processdifficulties, and yield losses associated with such processes.

The sensor element release process is simple, and may be done in thesame equipment as the sensor delineation process. It does not requireadditional expensive capital equipment or difficult processes at therelease step. No liquid or vapor undercuts are required and thus, nostiction results from wet undercut processes. In one embodiment, thesensor element is delineated using a DRIE trench etch, and then isreleased using a DRIE lateral etch. The lateral etch may be done as afooter etch in a DRIE machine.

The getter implant module getters defects in the device layer accordingto one embodiment, thus increasing the CMOS yield. According to otherembodiments, the process may not include a getter implant module.According to a further embodiment, the bottom of the epi device layer104 may be implanted with an appropriate getter implant prior to bondingthe device layer 104 onto the insulating layer 102 on the substrate 100.

The isolation process module provides anti-stiction bumps for lateralstiction prevention for moving structures such as rings and fingers.Other isolation and anti-stiction bumps may be formed during the sensorelement delineation and release process. A method of forming isolationand anti-stiction bumps according to an isolation process according toanother embodiment is further illustrated in FIGS. 35-42.

Referring to FIG. 35, a method of forming anti-stiction bumps generallyincludes providing a starting material having a silicon substrate 100 onthe bottom with an oxide insulation layer 102 provided on top, and anepitaxial device layer 104 of silicon formed upon the layer of oxide102. Additionally, an oxide film 113 is formed on top of the silicondevice layer 104. Next, in the step illustrated in FIG. 36, trenches 112are formed in a desired shape extending through silicon device layer 104and oxide layer 113. The trenches 112 are formed extending down to thetop surface of the oxide insulation layer 102.

The side walls of trenches 112 in the silicon device layer are linedwith a dielectric layer 114, such as an oxide, as shown in FIG. 37.

Referring to FIG. 38, the next step in the process of forming theanti-stiction bumps includes filling the trenches 112 with a conformalmaterial 118, such as polysilicon or nitride. The conformal material 118on top of oxide layer 113 is then removed, leaving conformal material118 in the trenches 112 as shown in FIG. 39. The oxide 113 on the topsurface of the silicon device layer 104 is then removed, leaving theliner oxide 114 and the conformal material 118 in the planarizedtrenches 112. The trenches 112 provide electrical isolation betweendifferent portions of the epitaxial device layer 104. An oxide film 116is then formed upon the top surface of the device, including upon theplanarized top surfaces of the one or more trenches 112 as seen in FIG.40.

Referring to FIG. 41, the sensor device is shown following the step ofremoving the oxide film 116 about a portion of the sensor device so asto expose part of the trenches. Next, in FIG. 42 the epitaxial devicelayer is etched away so as to form a suspended member 22A and to providefor the trench having conformal isolation and anti-stiction membersextending therefrom, labeled 40. The extending member 40 extending fromtrench 112 forms one or more isolation and anti-stiction bumps. Theanti-stiction bumps 40 provide for lateral stiction prevent for movingstructures, such as structure 22A.

Sensors made from the process have no need for temperature compensationsince all the mechanical components of the sensor element, including thedrive, sense, and balance electrodes, the ring, springs, etc., are madeof single crystal silicon. Thus, no differences exist in thermalexpansion rates of the sensor element members.

The electrical isolation of portions of the device silicon layer may bemoved to the end of the process. The frit bonding may occur over theon-board electronics of the integrated sensor device without adverselyaffecting the functionality of the circuitry.

It will be understood by those who practice the invention and thoseskilled in the art, that various modifications and improvements may bemade to the invention without departing from the spirit of the disclosedconcept. The scope of protection afforded is to be determined by theclaims and by the breadth of interpretation allowed by law.

1. A motion sensor comprising: a support substrate; a silicon sensingring formed within and supported by the substrate and having a flexuralresonance; at least one drive electrode comprising drive capacitiveplates for applying electrostatic force on the ring to cause the ring toresonate; at least one sense electrode comprising sense capacitiveplates for sensing a change in capacitance indicative of the vibrationnodes of resonance of the ring so as to sense motion; a plurality ofsilicon support springs connecting the substrate to the ring, whereinthe support springs are located at an angle to substantially match amodulus of elasticity of the silicon support springs; and electricalconnection comprising at least one external tether in contact with thering.
 2. The sensor as defined in claim 1 further comprising at leastone balance electrode comprising balance capacitive plates for applyingelectrostatic force to the ring to balance vibratory motion of theresonance of the ring.
 3. The sensor as defined in claim 1, wherein thesensor is approximately one-eighth symmetric.
 4. The sensor as definedin claim 1, wherein the sensor senses changes in capacitance at nodelocations in response to a Coriolis force.
 5. The sensor of claim 1,wherein the sensor is capable of at least four node operation.
 6. Thesensor as defined in claim 1, wherein the sensor utilizes at leastfourth and fifth flexural nodes of said sense element.
 7. The sensor asdefined in claim 1, wherein the sensor is integrated with on-boardelectronic compensating circuitry.
 8. The sensor as defined in claim 1,wherein the support springs comprise a first spring portion located atan angle in the range of 20° to 25° with respect to crystallineorientation of the silicon, and a second spring portion oriented at anangle in the range of 65° to 70° with respect to the crystallineorientation of the silicon.
 9. The sensor as defined in claim 8, whereinthe first spring portion is oriented at an angle of approximately 22.5°,and the second spring portion is oriented at an angle of approximately67.5°.
 10. The sensor as defined in claim 8, wherein the first springportion is oriented at an angle of approximately 22.5° and the secondspring portion is oriented at an angle of approximately 67.5° from the Xand Y-axes of the sensor.
 11. The sensor as defined in claim 1, whereinthe motion sensor comprises an angular rate sensor.
 12. The sensor asdefined in claim 1, wherein the motion sensor comprises an angularacceleration sensor.
 13. The sensor as defined in claim 1, wherein themotion sensor comprises an angular position sensor.
 14. The sensor asdefined in claim 1, wherein the substrate is comprised of silicon. 15.The sensor as defined in claim 1, wherein the substrate is comprised ofsilicon-on-insulator.
 16. The sensor as defined in claim 1, wherein theat least one drive electrode comprises at least two drive electrodes,and the at least one sense electrode comprises of at least two senseelectrodes.
 17. The sensor as defined in claim 2, wherein the at leastone balance electrode comprises at least two balance electrodes.
 18. Thesensor as defined in claim 1 further comprising a compensation massprovided on the ring to compensate for a modulus of elasticity variationaround the ring.
 19. The sensor as defined in claim 18, wherein thecompensation mass is located at locations at approximately 45° from theX and Y-axes of the sensor.
 20. The sensor as defined in claim 19,wherein the compensation mass is located at locations at approximately45° with respect to the crystal planes in the substrate.
 21. The sensoras defined in claim 1, wherein the silicon sensing ring has aperiodically varying width around its circumference.
 22. The sensor asdefined in claim 1, wherein the springs comprise a first radial portionand a second portion so as to have all spring portions located at anangle to substantially match a modulus of elasticity of the silicon. 23.The sensor as defined in claim 22, wherein the springs comprises atleast sixteen springs.
 24. The sensor as defined in claim 22, whereinthe springs are formed as pairs of springs.
 25. The sensor as defined inclaim 1, wherein the plurality of support springs are center mounted.26. The sensor as defined in claim 1, wherein the silicon sensing ring,the at least one drive electrode, the at least one sense electrode, theplurality of silicon support springs, and the at least one externaltether, is formed from and within the substrate.
 27. The sensor asdefined in claim 1, wherein the sensor comprises one or moreanti-stiction bumps formed to prevent stiction between a movingcomponent and another component of the sensor.
 28. The sensor as definedin claim 1, wherein one or more components of the sensor are perforated.29. The sensor as defined in claim 18, wherein said ring and thecompensating mass are perforated to comprise at least the compensatingmass.
 30. The sensor as defined in claim 1, wherein one or morecomponents of the sensor have corner compensation.
 31. The sensor asdefined in claim 2, wherein the at least one drive electrode, the atleast one sense electrode and the at least one balance electrode arecantilevered from the ring.
 32. The sensor as defined in claim 1,wherein the sensor has symmetric damping.
 33. The sensor as defined inclaim 1, wherein the at least one drive electrode receives adifferential drive signal and the at least one sense electrode generatesa differential sense signal.